In the making of gypsum wallboard wherein a stucco slurry is cast between front and back paper cover sheets, and dried in a kiln to convert the hemihydrate stucco into a dihydrate gypsum core, it is important that certain bonding take place between the cover sheets and the core. Failure to achieve this produces paper "blows" during the kiln heating, or "peelers" after the heating wherein the cover sheets peel cleanly from the core without adhering to any of the gypsum.
Attempts to avoid the above problems have led to a fairly complex but unproven, mechanical bonding theory. For decades, the accepted theory concerning the bonding of the cover sheet to the core has been that the gypsum mechanically attaches the paper by becoming dissolved in the paper while still in the hemihydrate slurry state. The dissolved gypsum was thought to thereafter form dihydrate crystals interlocked with the dihydrate core. According to the mechanical bonding theory, the absorption of water by the paper decreases the ratio of water to unset gypsum in the very thin interlocking layer of the core adjacent to the paper sheet, causing a different crystal growth in this area of imbalance. As a result, no bond is realized if the board is dried too fast or at high temperatures. This loss of bond was said to be due to the fact that the very thin crystals which form the bond are readily calcined and so lose their bond to the paper. And, to prevent this "stratification" and resultant "delamination", the solution for years has been to add starch to the gypsum slurry as a water retention agent for the sole purpose of protecting the gypsum crystals. The theroy has been that the water holding properties of the starch will inhibit the calcination of the gypsum crystals at the area of water imbalance in and adjacent to the sheet and hence protect the mechanical bond of the gypsum crystals to the paper. See, e.g., U.S. Pat. No. 2,044,401 issued on June 16, 1963. Substantially all prior board process patents are based solely on the mechanical bond theory.
It will be recognized that for such starch to function for bonding purposes, it must migrate to the paper-core interface from the slurry forming the core. Such migration is dependent upon the board core density, the water-resistance (sizing) of the paper, and the board drying conditions which include time, temperature, and humidity. It is definitely dependent on the starch being migratory and for this reason the starch used has most commonly been cooked, pregelatinized starch or one which has been dextrinized--a well-known degradation step which increases the cost of the starch component. Each of the above migration factors is subject to considerable variance at a board plant with the result that the production of satisfactory paper-core bonding is very unpredictable. A further, and considerable, disadvantage is that starch migration of necessity requires that a large portion of the starch will be "wasted", i.e., left in the core. For example, as much as 16 pounds of migratory starch per thousand square feet of board may be necessary to achieve sufficient migration to give proper bond. The result is that a significant amount, namely at least 90% as I have discovered, of an already costly component is wasted, resulting in a board which is more expensive than is necessary. It is because of this cost that the early history of starch addition to boards concerned methods of producing a cheaper starch additive.
Further prior art attempts have been made to improve, increase, or direct, in one fashion or another, the starch migration from the core to the paper-core interface. Examples include the addition of pregelatinized starch to the slurry, or the addition of a raw starch to the slurry where it is modified in situ by enzymes. However, these prior attempts almost uniformly are characterized by the addition of the starch to the slurry.
A recent innovation has been to add a form of starch to the bond liner prior to the casting of the slurry thereon. However, heretofore this technique absolutely required the use of pregelatinized or precooked starch, and even the step of roughening the bond liner. An example of this is shown in U.S. Pat. No. 3,516,882. Disadvantages arise from this technique, but these appear to have been ignored perhaps due to the continued reference in the 3,516,882 Patent to the old dihydrate bond theory. A most important disadvantage is that precooked starch migrates from the interface where it is desired. Still further, paper coated with precooked starch cannot be rewound, and must therefore be prepared at the board plant rather than at a paper mill. A further disadvantage is that precooked starch interferes with the wet bonding of the paper to the core, which wet bonding is essential to the invention as described below. Because of the wet bond interference which results when using precooked starch, various techniques such as the above-mentioned liner abrasion must be used in addition to make the precooked starch work at all.
Thus, rather than attempt to patch up piecemeal the old approach, an entirely new concept is needed.